1. Technical Field of the Invention
The present invention relates to magnetic random access memories, and more particularly to a magnetic random access memory array architecture which supports reduced write current needs and low voltage operation.
2. Description of Related Art
A magnetic random access memory (MRAM) element typically has a structure that includes a first and second magnetic layers which are separated by a non-magnetic layer. A magnetic vector in one of the two magnetic layers is magnetically fixed or pinned, while the magnetic vector of the other of the two magnetic layers is not fixed and thus its magnetization direction is free to be controlled and switched. Information is written to and read from the element as a logic “1” or a logic “0” (i.e., one or the other of two possible logic states) by changing the direction of the non-fixed magnetization vector in the other of the two magnetic layers. The differences in magnetization vector direction cause resistance variations within the element which can be measured. For example, the shifting of the magnetization vector direction can represent two different resistances or potentials, which are then read by the memory circuit as either a logic “1” or a logic “0.” The detection of these resistance or potential differences due to shifting magnetization vector direction allows information to be written to and read from the MRAM element.
Reference is now made to FIGS. 1A and 1B wherein there are shown schematic diagrams of conventional MRAM elements 10. Each element includes a bit line 12 and a word line 14. The memory storing structure of the element 10 is referred to as a “magnetic tunnel junction” 16 (MTJ) which is represented in the schematic by a variable resistance and is physically composed of the first and second magnetic layers and the separating non-magnetic layer discussed above.
With reference to FIG. 1A, one end of this resistance is connected to the bit line 12. The other end of the resistance is connected to a conduction terminal of an access transistor 18. The access transistor 18 in the illustrated element 10 is an n-channel FET with its source conduction terminal connected to ground and its drain conduction terminal connected to the other end of the resistance. The gate terminal of the access transistor 18 is connected to the word line 14.
With reference to FIG. 1B, one end of this resistance is connected to a reference voltage (for example, a ground reference). The other end of the resistance is connected to a conduction terminal of an access transistor 18. The access transistor 18 in the illustrated element 10 is an n-channel FET with its source conduction terminal connected to the bit line 12 and its drain conduction terminal connected to the other end of the resistance. The gate terminal of the access transistor 18 is connected to the word line 14.
In either of the embodiments of FIGS. 1A and 1B, a write digit line 20 (WDL) and a write bit line 22 (WBL) for the element 10 intersect at the magnetic tunnel junction 14. These lines 20 and 22 selectively carry currents and thus each selectively create a magnetic flux proximate to the magnetic tunnel junction 16. The magnetic fields induced by current flow in the lines 20 and 22 can be used to set the non-fixed direction of the magnetic vector within the magnetic tunnel junction 16. As discussed above, the setting of this direction affects the resistance of the magnetic tunnel junction 16. By selectively choosing the direction and magnitude of the current flow in the lines 20 and 22, one can program the magnetic tunnel junction 16, through its varying resistance, to store either one of two logic states: a logic “1” or a logic “0.” It is recognized, however, that the current in both the lines 20 and 22 must be of a certain magnitude in order to effectively control the non-fixed direction of the magnetic vector within the magnetic tunnel junction 16. It is accordingly imperative that sufficient current be made available in both lines 20 and 22 in order to write information into the element 10.
Reference is now made to FIG. 2 wherein there is shown a block diagram of a conventional MRAM memory array 50. The array 50 includes a plurality of individual MRAM elements 10 (of any suitable type including either of those shown in FIGS. 1A and 1B) arranged in a N×M array format. Each row 52 of elements 10 in the array 50 includes a word line 14 and a write digit line 20. Each column 54 of elements 10 in the array 50 includes a bit line 12 and a write bit line 22. Selection of a write digit line 20 and write bit line 22, along with the application of appropriate currents thereto, results in the writing of an information bit to the element 10 in the array 50 where the selected write digit line and write bit line intersect. Selection of a bit line 12 and a word line 14 turns on the access transistor 18 located at the intersection of the selected bit line and word line, and causes a current to flow through the magnetic tunnel junction 16 resistance whose magnitude is dependent on the programmed non-fixed direction of the magnetic vector within the magnetic tunnel junction. A sense amplifier (not shown) that is connected to the selected bit line 12 measures the current flowing in the bit line, as affected by the current flowing through the magnetic tunnel junction 16 resistance, in order to “read” the logic state of the element 10.
The write digit lines 20 and write bit lines 22 which extend across the rows and columns, respectively, of the array 50 are metal lines having a certain resistance which depends generally speaking on their metallic composition and dimensions (primarily length). The MRAM array 50 is typically supplied with a certain voltage (for example, 5V, 3.3V or 1.2V) which is fixed. When additional elements 10 are added to rows and/or columns of the array 50, the resistance of the individual write digit lines 20 and write bit lines 22 also increases. Ohm's Law, however, teaches that with a fixed voltage and an increasing resistance there is a corresponding decrease in the amount of current capable of being carried by each metal line. This presents a problem because, as discussed above, a certain magnitude of current is required in the lines 20 and 22 in order write information into the element 10. Increases in line 20 and line 22 length to accommodate additional rows/columns may preclude the lines 20 and 22, at the fixed supply voltage or at reduced voltage levels, from being able to carry sufficient programming currents. Thus, for a given fixed voltage and given line 20/22 characteristics, there exists a maximum line length which is permitted within the array 50 in order to ensure successful writing to an element 10.
The issue of sufficient current for programming the element 10 becomes of even greater concern when writing an entire word (for example, eight bits) into a memory location within the array 50 comprised of a corresponding plurality of elements. This operation requires that sufficient current be available for supply not only to the write digit line associated with the selected memory location, but also for supply simultaneously to the eight write bit lines associated with the elements 10 for that memory location. The potential division of available current among these multiple lines 20/22 for the word writing operation further restricts and limits the permitted lengths of the individual lines.
Several solutions have been proposed in the art to the foregoing line length limitation problem. One solution is to change the structure of the element 10, and perhaps also the technology used to fabricate it (for example, materials, layer deposition depth, and the like), so as to reduce the minimum current magnitude characteristic of the element 10. Experiments with alternative structures and/or fabrication techniques have not been successful. Another solution is to live with the line 20/22 length limitations and create larger sized memories by repeating sub-blocks formed of arrays 50 whose size is limited in the manner described above. This solution is not preferred as the overall area required for the memory unreasonably increases due to the need to repetitively include peripheral circuits (control logic, decoders, read/write circuits and the like) for each sub-block.
A need accordingly exists for a solution which would allow for increasing the number of rows or columns in an MRAM array without necessitating increases in supply voltage. Alternatively, a need exists for a solution which would allow for supply voltages to be decreased while continuing to maintain a certain number of rows and columns of elements.